Nearly 60 species of citrus trees have migrated to the rest of the world after abrupt changes to the climate, about 6 to 8 million years ago, according to new research. Long term, we can easily have a glass of fresh orange juice in our kitchen. To better understand where the orange trees came from, scientists studied the genomes of more than 50 citrus fruits. According to studies, they found that today’s citrus trees originate from at least 10 ancestral species found in the southwestern Himalayas, in the eastern region of Assam, north of Myanmar and west of Yun Nan (a province in China).
When the mountain ranges were high enough for climate change, about a million years ago, as the monsoon winds weakened, the humidity decreased and the climate dried up. This led to the spread of citrus trees from Southeast Asia, where new species evolved to adapt to different habitats.
Citrus trees are now widely planted among fruit trees worldwide, but it is not yet clear how they spread around the world. Fortunately, genetic science can reveal information about citrus history. But today’s research not only provides a history of how citrus trees spread over time, it can also provide a genetic roadmap for growers to create new varieties of citrus fruits that have good taste and different pests.
Scientists have long known that citrus trees originally came from Asia. Fossils of a citrus leaf in southwestern China, for example, show that common ancestors existed in the region about 8 million years ago. To answer their questions, the researchers analyzed the genomes of 58 existing citrus species, such as Mexican lemons, kamkats, and Cleopatra mandarins.
They also use two species close to each other that are not in the citrus group in order to understand in what order the genetic changes have occurred. It is interesting to know that the genes of these fruits tell a unique story in the same area where fossil leaves were found. About eight to ten million years ago, the ancestral citrus forest grew at the base of the Himalayas, but later changed. Citrus trees spread rapidly to Southeast Asia and from there to other parts of the world, including Australia some 4 million years ago.
From this mass migration over time, at least 10 ancestral citrus species appeared. Crossing all of these species in different parts of the world eventually produced many modern fruits. For example, today’s oranges are a combination of two ancestral species, including wild mandarin, which is usually small and sour, and wild pomelo, which is large and has very thick skin. Researchers believe it is unclear whether this mixture was naturally obtained or whether it was done by some skilled farmers who may have known how to do it thousands of years ago.
As these fruits spread to other parts of the world through commercial routes, new varieties were created. This information can also help growers in a very precise way as citrus fruits are threatened by a variety of diseases. For example, citrus greening, one of the most serious citrus diseases in the world, has destroyed millions of hectares of agricultural produce in the United States. Now the genetic map of many citrus fruits can help researchers identify which fruits are resistant to pests and also have a sweet and juicy taste.
Now it may be fun to know that a sweet orange was originally grown in the Himalayas millions of years ago and is now available to us.